A sequential circuit is a type of logic circuit that stores information about past inputs and uses it to determine the current output. The output of a sequential circuit depends on the present value of its input signals and on the sequence of past inputs, the input history. This is in contrast to combinational logic, whose output is a function of only the present input. Sequential circuits are used to construct finite-state machines, which are a basic building block in all digital circuitry. Virtually all circuits in practical digital devices are a mixture of combinational and sequential logic.
Sequential circuits consist of input variables, logic gates (or computational circuits), and output variables. They can be synchronous or asynchronous. Synchronous sequential circuits are designed to have an infinite number of states and are typically used to implement timers, counters, and memory elements. Asynchronous sequential circuits are similar to combinational circuits with feedback.
A familiar example of a device with sequential logic is a television set with "channel up" and "channel down" buttons. Pressing the "up" button gives the television an input telling it to switch to the next channel above the one it is currently receiving. However, if the television is on channel 8, pressing "up" switches it to channel 9, and if it is on channel 99, pressing "up" switches it to channel 2. This is because the television has stored information about the previous channel and uses it to determine the current output.
